In accordance with the Television Decoder Circuitry Act of 1990, all television sets sold in the United States that are 13" or larger in size must include capability for decoding closed caption (CC) information that is included in a data signal component of many television signals. The CC data signal component comprises two bytes of data during each occurrence of line 21 of field 1 of the television signal. A CC decoder extracts the data from the CC data signal component. The video processing portion of the television receiver produces an output video signal that includes both video program information and information corresponding to the CC data. A portion of a displayed image produced in response to the output video signal includes text corresponding to the CC data. The CC data may represent the audio program portion of a television program in which case the displayed text provides a visible representation of the audio program.
Other types of data components may also be included in a television signal in a form suitable for decoding using a decoder such as a closed caption decoder. For example, extended data services (XDS) data such as time of day information, program title, etc. may be included in line 21 of field 2 of the television signal in accordance with EIA 608 specifications. Also, teletext data and Starsight.RTM. program guide data may be included in various horizontal line intervals during vertical blanking such as line 16. The term auxiliary data as used herein includes any type of data component of a television signal that can be extracted using a decoder such as a closed caption decoder, including closed caption, XDS, teletext and Starsight.RTM. data. Similarly, the term auxiliary data decoder as used herein includes decoders for decoding the various types of auxiliary data described above.
To accurately decode auxiliary data, an auxiliary data decoder must be timed to operate during the portions of the horizontal display intervals that include auxiliary data, e.g., the latter half of line 21 of field 1 for closed caption data. A television receiver produces various timing signals that indicate when horizontal display intervals occur. For example, separated horizontal sync signals are produced in response to a synchronization signal component of the television signal. Also, horizontal deflection signals are generated to control electron beam deflection functions of an image display device, such as a kinescope.
Controlling an auxiliary data decoder with a deflection signal may increase the reliability of the decoded data. Decoder features, such as horizontal line and delay counters controlled by pulses in a sync signal, may count incorrectly in response to noise pulses in the sync signal causing the decoder to operate improperly. A sync signal that is derived from the television signal may include noise pulses corresponding to noise in the television signal. In comparison, deflection signals are generated using phase-locked loop (PLL) circuits that produce stable, uniform amplitude pulse waveforms that are less likely to include noise effects. Thus, it may be desirable to control an auxiliary data decoder using deflection signals.